1. Field of the Invention
The invention relates to a positioning device for positioning a measuring element. Furthermore, the invention relates to a measuring device with at least one measuring element for determining and/or monitoring a process variable and with a positioning device for positioning the measuring element. The process variable is, for example, the fill level or the temperature of a medium or a measured object.
2. Description of Related Art
Positioning devices of the aforementioned type are known, for example, from so-called 2.5D coordinate measuring devices. Optical or microwave-based 2.5D coordinate measuring devices known from the prior art are used for contactless measurement of a process variable to be determined at one level so that additional information is obtained for each coordinate of the level. The process variables are, for example, the height perpendicular to the measured level—also signified as fill level—or the respective temperature. Generally, a measured object or a measured medium (such as a liquid or a bulk material) is measured. The identified or monitored or, respectively, general measured process variable represents the half dimension of 2.5-dimensional process as additional information. The positioning of the measuring device or of the measuring element used in each case relative to the measurement level refers to the other two dimensions of the process.
Applications are, for example, material and substance testing in order, inter alia, to check geometrical dimensions of molded parts. Process automation permits such measurement as scanning of the surface of bulk materials in silos, thereby enabling a surface profile reconstruction and a more accurate determination of the fill level.
As a method for measuring the fill level, and thus, determining the amount relative to the measured level, for example, measurement using the radar method is known. The transit time method used by the measuring device is based on the physical laws that the path, for example, of an electromagnetic signal is equal to the product of transit time and velocity of propagation. In the case of measuring the fill level of a medium—for example, a liquid or a bulk material—in a container, the path corresponds to twice the distance between an antenna, which radiates the electromagnetic signal and receives it again, and the surface of the medium. The fill level can be calculated from the difference between the known distance between the antenna and the base of the container and the distance determined by the measurement of the surface of the medium to the antenna. The transmitted and received electromagnetic signals are mostly microwave radiation. The measurement itself, in order to obtain the value of the process variable at the respective selected location in the plane by positioning, can be accomplished by known and tested devices.
The main problem lies in the positioning of the measuring device, and in particular, of the measuring element, which must be aligned, in each case, relative to the measurement plane. In the mentioned measuring devices, which determine the fill level according to the radar method, for example, the antenna for radiating or receiving signals is to be aligned in a suitable manner to different areas.
Here, the positioning accuracy and the reproducibility are of great importance. It is known from the prior art to organize two stepping motor paths at right angles to one another (as so-called XY-cross tables or as XY-support, see, for example, European Patent EP 1 267 425 B1 and corresponding U.S. Pat. No. 6,765,335), so that a scanning of the Cartesian measurement plane is made. In some uses, the measured object is traversed and in others, the measuring element is traversed.
Another possibility is the combination of so-called rotary tables with a uni-axial linear positioning unit. The values of the process variable are recorded via polar coordinates.
In the prior art, a mirror arrangement is often used in the laser measuring method, wherein at least one mirror can be deflected in two directions in space. For the deflection of the mirror or the mirrors, at least two driving elements are also required.
Overall, positioning in the prior art is elaborate, complex, and thus associated with high costs.